In addition to the supermarket's own Gift Certificates, many now sell gift cards for other stores. The Safeways here in DC sell cards for gas stations, Best Buy, Olive Garden, etc. In a pinch at Xmas, I bought a few and was surprised to see them just added to the total as a purchase (not cash advance).

So, you can take it a step farther, and buy them for stores where you would shop anyway and get some extra points. Of course if you would be using an affinity CC, then it is a wash.